Electron guns are used to generate a directed stream of electrons with a predetermined kinetic energy. Electron guns are most commonly used to generate electron beams for vacuum tube applications such as cathode ray tubes (CRTs) found in televisions, game monitors, computer monitors and other types of displays.
Many medical and scientific applications require the generation of electron beams as well. Electron guns provide the electron source for the generation of X-rays for both medical and scientific research applications, provide the electron beam for imaging in scanning electron microscopes, and are used for microwave generation, e.g., in klystrons.
In many cases, the electron gun is incorporated into a linear accelerator system, or LINAC. LINACs have many industrial applications, including radiation therapy, medical and food product sterilization by irradiation, polymer cross linking and nondestructive testing (NDT) and inspection.
In addition, an electron gun is a key component of the injector system of many high-energy particle accelerator systems. The creation of high average-current, high brightness electron beams is a key enabling technology for these accelerator-based systems, which include high-energy LINACs such as Energy-Recovery LINAC (ERL) light sources, electron cooling of hadron accelerators, high-energy ion colliders, and high-power free-electron lasers (FELs). For these applications, the electron gun generates and provides a charged particle beam for input to the accelerator. The output of the accelerator system is an accelerated beam at the energy required for the particular application.
An electron gun, also referred to as an injector, is composed of at least two basic elements: an emission source and an accelerating region. The emission source includes a cathode, from which the electrons generated in the emission source escape. The accelerating region accelerates the electrons in the presence of an electric field to an accelerating electrode (anode), typically having an annular shape, through which the electrons pass with a specific kinetic energy. The commonly known cathodes used in electron guns generate electrons either by thermionic emission, field emission, or photoemission.
Photoemission cathodes typically generate a large number of electrons by photoemission from a laser-illuminated photocathode. The accelerated electrons typically enter an accelerating structure to reach higher energy. A high-current electron beam is thus generated at an output port of the injector of a high-power accelerator.
Very high average current electron injectors are required for a number of applications. The amplitude of the current is determined by the quantum efficiency (QE) of the cathode and the power of the laser beam available. Hence, the obvious choice for these applications is a high QE cathode irradiated by the highest power of the laser available. However, there are inherent problems with this approach. The high QE cathodes are typically sensitive to contamination and thus have a very limited lifetime. Furthermore, the commercially available lasers do not have enough power to deliver the average currents required from these cathodes for some of these applications.
A reliable, efficient, long-life high power laser and photocathode combination capable of generating high-current low-emittance electron beams has recently been disclosed in commonly owned U.S. Pat. Nos. 7,227,297 and 7,601,042 to Srinivasan-Rao et al., (“the Srinivasan-Rao patents”), the specifications of which are incorporated herein by reference in their entireties for all purposes. The electron gun device disclosed in these patents includes a secondary emitter that emits secondary electrons in response to receiving a beam of primary electrons. In one mode, the primary beam of electrons is generated by photoemission from the photocathode in response to a laser beam striking the photocathode.
In a preferred embodiment, the Srinivasan-Rao patents propose using an encapsulated secondary emission enhanced cathode device, which contains the photocathode and the secondary emitter in a vacuum within a housing. The photocathode includes a primary emission surface adapted to emit primary electrons from the primary emission surface. The housing defines a drift region through which the primary electrons are accelerated to a desired energy. The secondary emitter has a secondary emission surface that has negative-electron-affinity. The secondary emission surface emits secondary electrons in response to primary electrons impinging on the secondary emitter.
The Srinivasan-Rao patents further disclose use of one of single crystal diamond, polycrystalline diamond, and diamond-like carbon for the non-contaminating secondary emitter. It has been found that such a diamond amplified photocathode can perform multiple functions: 1) It amplifies the primary current from a conventional photocathode with amplification factors exceeding 200, thereby reducing the demands on the primary cathode and the laser; and 2) It also acts as a window that isolates the cathode from the RF cavity, thereby shielding them from contaminating each other.
However, while the general concept of an encapsulated secondary emission enhanced cathode device has been proposed, attempts to successfully commercially fabricate such devices have proven quite difficult and a specific optimum structure for such a device has heretofore been unknown. For example, earlier attempts at forming such encapsulated composite structures involved brazing techniques. However, brazing diamond to other materials such as niobium (Nb) was met with limited success. Specifically, the high temperatures required in brazing were incompatible with the diamond preparation and K2CsSb cathode fabrication.
Accordingly, it would be desirable to provide an encapsulated secondary emission enhanced cathode device for use in an electron gun, which is easily and reliably manufactured. It would be further desirable to provide such a cathode device having an optimum non-contaminating structure, which permits simple and reliable manufacture and which will efficiently operate in superconducting RF electron guns for the generation of high-current high-brightness electron beams.